Economic model for any market dominated by a small number of firms. Generally, most markets tend towards an oligopoly so it is considered one of the most important. The main assumptions for an oligopoly are:-

supply concentrated in the hands of a few firms, although there can be any number of firms in the industry.

Non-price competition - As the product in the market are not homogeneous, there is some brand loyalty. Therefore promotion of brands through advertising is essential for the major firms. For instance, Mars has to spend much on marketing to try and differentiate it's chocolate products from those of Cadbury. This will often mean a higher priced branded goods sell better than cheaper substitutes.

Price rigidity - self explanitary, really.

L-shaped cost curves - The variable costs over a large range of output have been observed to be constant, meaning cost curves deviate from their usual U-shapes.

Collusion - If oligopolistic firms collude, they can act like a monopoly, thus increasing abnormal profits. Although this is illegal in most cases it is still common.